Transmission systems are a vital link in the powertrain of any vehicle. The purpose of the transmission system is to employ gears of various sizes to provide an engine with a mechanical advantage over the driving wheels. Maintaining lubrication to the moving parts within the transmission helps to ensure proper operation and long-lasting transmission life. Significant problems often result if moving parts within a transmission are not sufficiently lubricated. Proper lubrication flow characteristics are also important to long-term transmission operation. If inadequate lubricant flows to moving parts, they may prematurely wear, leading to failure.
Additionally, wear of a single moving part within a transmission may produce harmful debris such as metal filings that may interfere with and damage other moving parts within the transmission. While the majority of the debris in the transmission are encountered in the first several thousand miles of operation, activities such as the clutching of teeth can cause such debris to exist over the life of the unit. Moreover, the mere existence of debris may significantly shorten the useful life of a transmission, thereby requiring costly and inconvenient replacement. Finally, the problem of debris formation is especially significant in transmissions that experience high torque loading, such as those used in heavy commercial vehicles.
In the past, transmission wear was reduced by adding multiple counter-rotating shafts with each bearing only a fraction of the transmission load, so that one shaft need not bear the entire load. Transmissions of this type, containing two or more countershafts having radially aligned gears on each of the countershafts that simultaneously engage and support a gear encircling a main shaft of the transmission, are well known, and are generally described in U.S. Pat. Nos. 3,105,395 and 3,237,472. Multiple countershaft transmissions allow the delivery of increasingly high torque and correspondingly higher loads while reducing stress on given gears. However, as torque levels have continually increased, such that transmissions are required to endure increased loads, degradation of the gears and especially of the countershaft bearings has become a problem. In particular, as torque on the countershafts has exceeded 2,000 lb-ft, and even approached 2,200 to 2,400 lb-ft of torque, gear and bearing degradation has become severe enough that transmission life is significantly reduced.
Conventionally, as noted above, multiple countershaft transmissions include a lubricating transmission fluid generally collected in a pan on the lower side of the transmission. In some transmissions, a submerged oil pump moves a portion of the lubricating transmission oil through a flow path to directly or forcibly lubricate countershaft synchronizers to extend the synchronizer life. However, other components within the transmission, and in particular the gears and the countershaft bearings, are generally lubricated by "splash" lubrication within the transmission, whereby oil residing in the oil pan and the rest of the transmission is agitated and splashed within the transmission casing. In known transmissions experiencing torque less than about 2000 lb-ft exerted upon the shaft, such splash lubrication has proven sufficient for extended lifetime. However, as torque levels have increased, the gears and the bearings experience significantly higher stress that is not alleviated through conventional splash lubrication.
Moreover, with increased torque levels, engine lower gears, located adjacent the front of the transmission casing, have increased in size. In general, as gear size has increased, places such as the pockets holding the bearings that support the front of a rotating countershaft become shielded by the gears and receive insufficient splash lubrication. As a result, front countershaft bearings prematurely wear and degrade, thereby limiting the transmission life or increasing the frequency of repair. Moreover, one set of gears may shield other sets of gears, so that some gears may be adequately lubricated while others are not. The shielded gear sets may then pit and degrade, producing harmful debris within the transmission. Accordingly, one way to improve high torque transmissions and to retain gear integrity to extend transmission life is to ensure adequate lubrication to all gears and to all front countershaft bearings.